This invention relates to a switching power supply circuit which includes a circuit for improving the power factor.
Various power supply circuits wherein a resonance type converter is provided on the primary side have been proposed by the assignee of the present patent application.
FIG. 9 shows an example of a switching power supply circuit including a configuration for improving the power factor which is one of switching power supply circuits proposed by the assignee of the present patent application. The switching power supply circuit is suitable for conditions of the load power Po=200 W or more and the ac input voltage VAC=200 V type or conditions of the load power Po=150 W or less and the ac input voltage VAC=100 V type.
Referring to FIG. 9, the power supply circuit shown includes a common mode choke coil CMC and a filter capacitor CL connected in such a manner as seen in FIG. 9 to a commercial ac power supply AC to form a filter for removing harmonics superposed on the commercial ac power supply AC.
A power choke coil PCH is inserted in series in one of a pair of lines of the commercial ac power supply AC. The power factor PF is improved to approximately 0.75 by the power choke coil PCH.
A full-wave rectification circuit including a bridge rectification circuit Di and a smoothing capacitor Ci connected in such a manner as shown in FIG. 9 is formed for the commercial ac power supply AC. The full-wave rectification circuit rectifies and smoothes the commercial ac power supply AC to produce a rectified smoothed voltage Ei which appears across the smoothing capacitor Ci. The rectified smoothed voltage Ei has a level equal to the ac input voltage VAC and is inputted as a dc input voltage to a primary side switching converter of the following stage.
In this instance, a current resonance type separately excited converter is used as the switching converter which performs a switching operation with the above-mentioned dc input voltage inputted thereto. The current resonance type converter includes two switching elements Q1 and Q2 as seen in FIG. 9.
In this instance, the switching elements Q1 and Q2 are formed from MOS-FETs and connected in such a manner as seen in FIG. 9 to form a switching circuit of the half bridge coupling type.
Clamp diodes DD1 and DD2 are connected in such directions as seen in FIG. 9 in parallel to the switching elements Q1 and Q2, respectively.
A partial resonance capacitor Cp for partial voltage resonance is connected to the switching element Q2 from between the switching elements Q1 and Q2.
An isolation converter transformer PIT is provided to transmit a switching output of the primary side switching converter to the secondary side.
The isolation converter transformer PIT includes, for example, an EE type core shown in FIG. 13. A primary winding N1 and a secondary winding N2 are wound on a central magnetic leg of the EE type core of the isolation converter transformer PIT using a bobbin or the like such that an isolation condition from each other may be assured.
The central magnetic leg of the EE type core has a gap of, for example, approximately 1.5 mm to 2.0 mm formed therein so that a loose coupling state wherein the coupling coefficient k is approximately k=0.8 may be obtained between the primary winding N1 and the secondary winding N2. This prevents occurrence of abnormal vibrations when an intermediate load is applied.
The primary winding N1 of the isolation converter transformer PIT is connected at one end thereof to the drain of the switching element Q1 and at the other end thereof to a source-drain node of the switching elements Q1 and Q2 through a series resonance capacitor C1. Through the connection just described, a switching output of the switching elements Q1 and Q2 is transmitted to the primary winding N1.
In the connection scheme described, the primary winding N1 and the series resonance capacitor C1 are connected in series, and thus, a primary side series resonance circuit is formed from the leakage inductance of the primary winding N1 and the capacitance of the series resonance capacitor C1. The primary side series resonance circuit makes the switching operation of the switching elements Q1 and Q2 a switching operation of the current resonance type.
A full-wave rectification circuit formed from a bridge rectification circuit DBR and a smoothing capacitor C0 is connected to the secondary winding N2 of the isolation converter transformer PIT. A secondary side dc output voltage E0 is obtained across the smoothing capacitor C0 by the full-wave rectification circuit. The secondary side dc output voltage E0 is supplied to a load not shown. Further, the secondary side dc output voltage E0 is branched and supplied as a detection voltage also to an oscillation drive/control circuit 2.
The oscillation drive/control circuit 2 may be formed typically from an IC for universal use and is provided to drive the switching elements Q1 and Q2 in accordance with separate excitation system to perform a switching operation.
Driving signals (voltages) are outputted from the oscillation drive/control circuit 2 to the gates of the switching elements Q1 and Q2 so that the switching elements Q1 and Q2 perform switching on/off alternately with a required switching frequency.
The oscillation drive/control circuit 2 operates to vary the frequency of the driving signals in response to the level of the secondary side dc output voltage E0 inputted thereto. Consequently, the switching elements Q1 and Q2 are controlled so as to vary the switching frequency in response to the level of the secondary side dc output voltage E0.
When the switching frequency varies in this manner, the resonance impedance of the primary side dc resonance circuit varies, and also the energy to be transmitted from the primary side to the secondary side in the isolation converter transformer PIT varies. Therefore, also the level of the secondary side dc output voltage E0 is variably controlled. In other words, the secondary side dc output voltage E0 is varied by variably controlling the switching frequency thereby to achieve constant voltage control.
The power supply circuit shown in FIG. 9 is suitable for conditions of the load power Po=200 W or more and the ac input voltage VAC=200 V type or conditions of the load power Po=150 W or less and the ac input voltage VAC=100 V type. In contrast, in order to satisfy conditions of the load power Po=200 W or more and the ac input voltage VAC=100 V type, the rectification circuit system for rectifying the commercial ac power supply AC to obtain the rectified smoothed voltage Ei (dc input voltage), the power supply circuit shown in FIG. 9 is modified in such a manner as seen in FIG. 10, in which like reference characters to those of FIG. 9 denote like elements.
Referring to FIG. 10, the power supply circuit shown includes, as a rectification circuit system for rectifying the commercial ac power supply AC, two rectification diodes D13 and D14 and two smoothing capacitors Ci1 and Ci2. The elements mentioned are connected in such a manner as seen in FIG. 10 so that the rectified smoothed voltage Ei (dc input voltage) obtained across the smoothing capacitors Ci1 and Ci2 connected in series has a level equal to twice that of the ac input voltage VAC. In other words, the rectification circuit system is formed as a double voltage rectification circuit.
It is known that, for example, where the dc input voltage is equal to twice the ac input voltage VAC in a comparatively low load condition that the load power Po is Po=200 W or more where the ac input voltage VAC is 100 V type, the peak current flowing through the switching elements in the succeeding stage increases and the power loss increases as much. Therefore, if the dc input voltage of the level equal to twice that of the ac input voltage VAC is obtained by the double voltage rectification circuit shown in FIG. 10, then the level of the peak current to flow through the switching circuits can be suppressed.
FIG. 11 shows a further power supply circuit which includes a power factor improving circuit for improving the power factor for a voltage resonance type self-excited switching converter.
The power supply circuit is configured such that a power factor improving rectification circuit 20 for improving the power factor is provided for a converter circuit which includes a combination of a half bridge coupling current resonance type converter and a partial voltage resonance circuit with which voltage resonance occurs only upon turning off of a semiconductor switch.
In the power supply circuit shown in FIG. 11, ac input current IAC is rectified by the power factor improving rectification circuit 20 (described later) and smoothed by the two smoothing capacitors Ci1 and Ci2 connected in series so that a rectified smoothed voltage Ei by a double voltage rectification system equal to twice that obtained by the full-wave rectification system is obtained.
The power supply circuit further includes a self-excited current resonance type converter which uses, as a power supply for operation, the rectified smoothed voltage Ei which appears across the smoothing capacitors Ci1 and Ci2.
In the current resonance type converter, two switching elements Q1 and Q2 each in the form of a bipolar transistor are half-bridge coupled as seen in FIG. 11 and inserted between the positive electrode side of the smoothing capacitor Ci1 and the primary side ground.
Starting resistors RS1 and RS2 are inserted between the collector and the base of the switching elements Q1 and Q2, respectively. Resistors RB1 and RB2 are connected to the bases of the switching elements Q1 and Q2 and set base current (drive current) of the switching elements Q1 and Q2, respectively.
Clamp diodes DD1 and DD2 are inserted between the base and the emitter of the switching elements Q1 and Q2, respectively. The clamp diodes DD1 and DD2 form current paths for clamp current which flows between the base and the emitter within periods within which the switching elements Q1 and Q2 are off, respectively.
Resonance capacitors CB1 and CB2 cooperate with driving windings NB1 and NB2 of a drive transformer PRT described below to form a series resonance circuit for self-excited oscillation (self-excited oscillation drive circuit) and determine the switching frequency of the switching elements Q1 and Q2.
The drive transformer PRT (Power Regulating Transformer) drives the switching elements Q1 and Q2 and variably controls the switching frequency to perform constant voltage control. In the circuit shown in FIG. 11, the driving windings NB1 and NB2 on the drive transformer PRT are wound while a control winding NC is wound in an orthogonal direction to that of the driving windings NB1 and NB2 thereby to form an orthogonal saturable reactor.
One end of the driving winding NB1 of the drive transformer PRT is connected to the base of the switching element Q1 through a series connection of the resistor RB1 and the resonance capacitor CB1. The other end of the driving winding NB1 forms a tap point connected to a resonance current detection winding ND and is connected to the emitter of the switching element Q1.
One end of the driving winding NB2 is grounded, and the other end of the driving winding NB2 is connected to the base of the switching element Q2 through a series connection of the resistor RB2 and the resonance capacitor CB2.
The driving winding NB1 and the driving winding NB2 are wound such that they generate voltages of the opposite polarities to each other.
The isolation converter transformer PIT (Power Isolation Transformer) transmits switching outputs of the switching elements Q1 and Q2 to the secondary side.
In the isolation converter transformer PIT, a gap is formed in a central magnetic leg of an EE type core similarly as in FIG. 13, and the primary winding N1 and the secondary winding N2 are wound in an isolated relationship from each other using a bobbin.
One end of the primary winding N1 of the isolation converter transformer PIT is connected to a node (switching output point) between the emitter of the switching element Q1 and the collector of the switching element Q2 through the resonance current detection winding ND so that a switching output can be obtained therefrom.
The other end of the primary winding N1 is connected to a node between high speed recovery type diodes D1 and D2 in the power factor improving rectification circuit 20 through the series resonance capacitor C1.
In this instance, the series resonance capacitor C1 and the primary winding N1 are connected in series, and a primary side current resonance circuit for making operation of the switching converter operation of the current resonance type is formed from a capacitance of the series resonance capacitor C1 and a leakage inductance component L1 of the isolation converter transformer PIT including the primary winding N1 (series resonance winding).
A parallel resonance capacitor Cp is connected in parallel between the collector and the emitter of the switching element Q2.
Through the connection of the parallel resonance capacitor Cp, a voltage resonance operation is obtained only upon turning off of the switching elements Q1 and Q2 by the capacitance of the parallel resonance capacitor Cp and the leakage inductance component L1 of the primary winding N1. In short, a partial voltage resonance circuit is formed.
On the secondary side of the isolation converter transformer PIT shown in FIG. 11, a center tap is provided for the secondary winding N2 and rectification diodes D01, D02, D03 and D04 and smoothing capacitors C01 and C02 are connected in such a manner as seen in FIG. 11 to form two full-wave rectification circuits including a full-wave rectification circuit of the rectification diodes D01 and D02 and the smoothing capacitor C01 and another full-wave rectification circuit of the rectification diodes D03 and D04 and the smoothing capacitor C02. The full-wave rectification circuit including the rectification diodes D01 and D02 and the smoothing capacitor C01 produces a dc output voltage E01 while the full-wave rectification circuit including the rectification diodes D03 and D04 and the smoothing capacitor C02 produces another dc output voltage E02.
It is to be noted that, in this instance, the dc output voltage E01 and the dc output voltage E02 are branched and inputted also to a control circuit 1. The control circuit 1 utilizes the dc output voltage E01 as a detection voltage and uses the dc output voltage E02 as a power supply for operation of the control circuit 1.
The control circuit 1 supplies dc current, whose level is varied, for example, in response to the level of the dc output voltage E01 of the secondary side, as control current to the control winding NC of the drive transformer PRT to perform constant voltage control.
The power supply circuit having the configuration described above performs a switching operation in the following manner. First, when a commercial ac power supply is made available, starting current is supplied to the bases of the switching elements Q1 and Q2, for example, through the starting resistors RS1 and RS2, respectively. If, for example, the switching element Q1 is turned on first, then the switching element Q2 is controlled so as to be turned off. Thus, as an output of the switching element Q1, resonance current flows along the resonance current detection winding NDxe2x86x92primary winding N1xe2x86x92series resonance capacitor C1, and the switching elements Q1 and Q2 are controlled so that, when the resonance current exhibits a value in the proximity of 0, the switching element Q2 is turned on while the switching element Q1 is turned off. Then, resonance current flows through the switching element Q2 now in the opposite direction. Thereafter, a self-excited switching operation wherein the switching elements Q1 and Q2 are turned on alternately is repeated.
When the switching elements Q1 and Q2 repeat opening and closing operations alternately using the terminal voltage of the smoothing capacitors Ci1 and Ci2 as a power supply for operation in this manner, drive current having a waveform proximate to a resonance current waveform is supplied to the primary winding N1 of the isolation converter transformer PIT thereby to obtain an alternating output at the secondary winding N2.
As described above, the control circuit 1 supplies the dc current, whose level is varied, for example, in response to the level of the dc output voltage E01 on the secondary side, as control current to the control winding NC of the drive transformer PRT to perform constant voltage control.
In particular, the control current corresponding to the level of the dc output voltage E01 is supplied to the control winding NC to vary the inductance values of the driving windings NB1 and NB2 thereby to vary the conditions of the self-excited oscillation circuit to control the switching frequency. Consequently, the switching frequency of the switching elements Q1 and Q2 is varied in response to the level of the dc output voltage E01 thereby to control the drive current to be supplied to the primary winding N1 of the primary side series resonance circuit to control the energy to be transmitted to the secondary side to achieve constant voltage control of the secondary side dc output voltage.
It is to be noted that the constant voltage control method according such a method as described above is hereinafter referred to as xe2x80x9cswitching frequency control methodxe2x80x9d.
Now, a configuration of the power factor improving rectification circuit 20 is described.
The power factor improving rectification circuit 20 has a power factor improving circuit configuration of the electrostatic coupling type power feedback type.
The power factor improving rectification circuit 20 has a rectification action for ac input current IAC and has a power factor improving action for the ac input current IAC.
In the power factor improving rectification circuit 20, a film capacitor is disposed as a capacitor CN for normal mode noise suppression between ac lines.
Further, the two high speed recovery type diodes D1 and D2 are provided through a choke coil (inductor L10).
The high speed recovery type diodes D1 and D2 are connected in series and disposed between the positive terminal of the smoothing capacitor Ci1 and the primary side ground.
The primary winding N1 of the isolation converter transformer PIT is connected to a node between the high speed recovery type diodes D1 and D2 through the series resonance capacitor C1.
Further, capacitors C21 and C22 are provided. The capacitor C22 is connected in parallel to the high speed recovery type diode D1 while the capacitor C21 is connected in parallel to the high speed recovery type diode D2.
In the power factor improving rectification circuit 20 having the configuration described above, the high speed recovery type diodes D1 and D2 function as a rectification circuit.
Within a period within which the ac input voltage VAC is positive, rectified current flows along the ac power supply ACxe2x86x92inductor L10xe2x86x92high speed recovery type diode D1xe2x86x92smoothing capacitor Ci1xe2x86x92. . . , whereby the smoothing capacitor Ci1 is charged.
Within another period within which the ac input voltage VAC is negative, rectified current flows along the ac power supply ACxe2x86x92inductor L10xe2x86x92smoothing capacitor Ci2xe2x86x92primary side groundxe2x86x92high speed recovery type diode D2xe2x86x92. . . , whereby the smoothing capacitor Ci2 is charged.
The smoothing capacitors Ci1 and Ci2 are connected in series and a rectified smoothed voltage Ei is extracted from the positive terminal side of the smoothing capacitor Ci1, thereby achieving the double voltage rectification system.
The power factor improving rectification circuit 20 has the following power factor improving function.
As described above, the current resonance circuit formed from the series resonance capacitor C1 and the primary winding N1 is connected to the node between the two high speed recovery type diodes D1 and D2. Further, the inductor L10 and capacitors C21 and C22 are connected to the node between the high speed recovery type diodes D1 and D2.
In this instance, by power feedback wherein primary side series resonance current is regenerated to the smoothing capacitors Ci1 and Ci2 through the inductor L10 and capacitors C21 and C22, the high speed recovery type diodes D1 and D2 perform a switching operation when the absolute value of the ac input voltage VAC is higher than xc2xd of its peak value.
Consequently, also within a period within which the rectified output voltage level is lower than the voltage across the smoothing capacitor Ci1 (or Ci2), charging current flows to the smoothing capacitor Ci1 (or Ci2).
As a result, an average waveform of the ac input current approaches the waveform of the ac input voltage and increases the continuity angle of the ac input current, and consequently, improvement of the power factor is achieved.
FIG. 12 illustrates, as a characteristic of the power supply circuit shown in FIG. 11, variations of the AC to DC power conversion efficiency (xcex7AC/DC), power factor PF and rectified smoothed voltage Ei with respect to a load variation where the power supply circuit is configured for the conditions of the load power Po=200 W and the ac input voltage VAC=100 V.
It is to be noted that a power choke coil PCH (L11) is inserted in one of the ac power supply lines and the value of the inductance L11 of the power choke coil PCH is set so that the power factor PF may exhibit a value of 0.75 when the load power Po is in the maximum as indicated by a solid line in FIG. 12.
Also the power supply circuit just described has the following problems.
Since the power choke coil PCH involves iron loss and copper loss and therefore exhibits some increased power loss and some drop of the dc input voltage, there is a problem that the AC to DC power conversion efficiency xcex7AC/DC drops.
Where the load power Po is Po=200 W, the inductance L11 of the power choke coil PCH is 4.4 mH and the power factor PF is PF=0.76, and a harmonic distortion regulation value is cleared. However, when compared with that of another case wherein the power choke coil PCH is not connected as indicated by a broken line in FIG. 12, due to power loss of the power choke coil PCH and a drop of the rectified smoothed voltage Ei to 13.5 V, the AC to DC power conversion efficiency xcex7AC/DC drops by 0.3% and the ac input power increases by 0.6 W.
Further, as the load power increases, the scale of the power choke coil PCH increases, resulting in increase of the weight, size and cost.
For example, the necessary weight of the power choke coil PCH is approximately 240 g, and the volume occupation is 48 cm3 and the mounting area on a printed circuit board is 19.2 cm2.
Further, the location of the power choke coil PCH must be selected so that leakage magnetic fluxes therefrom may not have a bad influence on any other element, or a countermeasure which prevents an influence of such leakage magnetic fluxes is required.
Accordingly, the location of the power choke coil PCH on a circuit board is restricted.
It is an object of the present invention to provide a switching power supply circuit which is improved in the power factor and the power conversion efficiency and reduced in size and weight.
In order to attain the object described above, according to an aspect of the present invention, there is provided a switching power supply circuit comprising:
rectification smoothing means including a rectifier and a smoothing capacitor connected in series for rectifying and smoothing an ac voltage supplied through two lines for an ac power supply;
an isolation converter transformer including a core and a primary winding, a secondary winding and tertiary winding wound on the core for transmitting an output on a primary side obtained by the primary winding to a secondary side wherein the secondary winding is wound, the tertiary winding being provided on the primary side;
switching means including two switching elements coupled in a half-bridge coupling for intermittently outputting an output voltage of the smoothing means to the primary winding of the isolation converter transformer;
switching driving means for driving the switching elements to perform a switching operation;
a current resonance circuit formed from a leakage inductance component of the primary winding of the isolation converter transformer and a capacitance of a series resonance capacitor connected in series to the primary winding for making the operation of the switching means operation of the current resonance type;
a partial voltage resonance circuit formed from a capacitance of a parallel resonance capacitor connected in parallel to one of the switching elements and a leakage inductance component of the primary winding of the isolation converter transformer for performing a voltage resonance operation within a turnoff period of each of the switching elements; and
dc output voltage production means for receiving and rectifying an alternating voltage obtained by the secondary winding of the isolation converter transformer to produce a secondary side dc output voltage,
wherein the tertiary winding is connected between one of the two lines for the ac power supply and the smoothing capacitor.
According to another aspect of the present invention, there is provided a switching power supply circuit comprising:
a rectification smoothing circuit including a voltage rectifier for rectifying ac current and a smoothing capacitor for smoothing the rectified current from the voltage rectifier;
an isolation converter transformer including a core and a primary winding, a secondary winding and tertiary winding wound on the core for transmitting an output on a primary side obtained by the primary winding to a secondary side wherein the secondary winding is wound;
switching means including two switching elements coupled in a half-bridge coupling for intermittently outputting an output voltage of the rectification smoothing circuit to the primary winding of the isolation converter transformer;
switching driving means for driving the switching elements to perform a switching operation;
a primary side series resonance circuit formed from a leakage inductance component of the primary winding of the isolation converter transformer and a capacitance of a primary side series resonance capacitor connected in series to the primary winding for making the operation of the switching means operation of the resonance type;
a partial voltage resonance circuit formed from a capacitance of a primary side partial resonance capacitor connected in parallel to one of the switching elements and a leakage inductance component of the primary winding of the isolation converter transformer for performing a voltage resonance operation within a turnoff period of each of the switching elements;
a power factor improving circuit including a first high speed recovery type diode element connected between the voltage rectifier and the smoothing capacitor, a series connection circuit of the tertiary winding and a second high speed recovery type diode element connected in parallel with the first high speed recovery type diode element; and
dc output voltage production means for receiving and rectifying an alternating voltage obtained by the secondary winding of the isolation converter transformer to produce a secondary side dc output voltage.
With the switching power supply circuits, improvement of the power factor of a circuit which includes a combination of a current resonance type converter of a switching frequency controlling system with a partial voltage resonance circuit where the load power is 150 W or more and the circuit employs an input double voltage rectification system is achieved by connecting a series resonance capacitor or inductor in series to a tertiary winding wound on the primary side of an isolation converter transformer so that the voltage is fed back to a rectification circuit composed of a high speed recovery type diode. Consequently, improvement of the power factor, improvement of the power conversion efficiency and reduction in size and weight are achieved.
The above and other objects, features and advantages of the present invention will become apparent from the following description and the appended claims, taken in conjunction with the accompanying drawings in which like parts or elements denoted by like reference symbols.